The present invention relates generally to methods for adding alloying ingredients to molten steel and more particularly to a method for adding bismuth to molten steel in a ladle.
Bismuth is added to steel as an alloying ingredient to improve the machinability of the steel. In a typical steelmaking operation, molten steel from a steel-making furnace is introduced into a ladle from which the molten steel is then introduced into ingot molds or, in the case of a continuous casting operation, into a tundish. Alloying ingredients may be added to the molten steel outside of the steel-making furnace, either in the ladle or as the steel is being introduced into an ingot mold or tundish.
Bismuth may be added to steel in the form of shot. Bismuth has been added to steel as the molten steel is introduced into an ingot mold, and bismuth may be added to molten steel as the latter is being introduced into a tundish. It is normally desirable to add alloying ingredients to the molten steel in the ladle because, in theory at least, when so added, the alloying ingredient should be more uniformly distributed throughout the entire volume of steel contained in the ladle than would be the case if the alloying ingredient were added to the same volume of steel as the latter was being introduced into the ingot molds or into the tundish. In other words, the result of a ladle addition should be an unchanging percentage of alloying ingredient in the molten steel, from the beginning until the end of the withdrawal of the molten steel from the ladle.
In the case of bismuth, however, a problem arises when bismuth is added to molten steel in the ladle. This problem, known as bismuth "fade", is reflected by a decreasing bismuth content in the molten steel during the time the molten steel is being withdrawn from the ladle. In other words, after the bismuth has been added, the steel withdrawn from the ladle at the beginning of the withdrawal or casting period has a higher bismuth content than the steel withdrawn from the ladle at the end of the casting period, and there is a declining percentage in the bismuth content from beginning to end of the casting period.
It has been determined that the problem of bismuth fade is due to a vaporization of bismuth at the surface of the molten steel in the ladle followed by an oxidation of the bismuth vapor producing a fume which is then carried away from the ladle by the exhaust system normally associated with the ladle. The bismuth vapor which has been oxidized and carried away by the exhaust system is replenished at the surface of the molten steel in the ladle by a transport of bismuth upwardly through the molten steel to the surface thereof. Bismuth transport probably occurs on an atomic level and constitutes a diffusion or migration of bismuth atoms upwardly through the molten steel to the surface thereof.
The amount of bismuth vapor is reflected by the partial pressure due to bismuth. The partial pressure due to bismuth, which is a measure of the tendency of bismuth to vaporize, increases with an increase in the temperature at the surface of the bismuth-containing molten steel.
When bismuth vapor is oxidized, the partial pressure due to bismuth is lowered. As this occurs there is a tendency for the bismuth contained within the molten steel to vaporize so as to compensate for the depletion of the bismuth vapor and attempt to maintain the partial pressure due to bismuth at the equilibrium level for the particular temperature then existing at the surface of the molten steel. However, the bismuth vapor above the molten steel is oxidized as fast as it is replenished from within the molten steel which in turn causes a gradual depletion of the bismuth content in the molten steel in the ladle, i.e. bismuth fade.